<!DOCTYPE html>
<html>
	<head>
		<meta charset="utf-8">
		<title>event</title>
	</head>
	<body>
		<button id="btn1" onclick="fun1()">click</button>
		<button id="btn2">click</button>
		<button id="btn3">click</button>
		<script>
			function fun1(){
				console.log("funtion 1");
			}
			btn2.onclick = fun2;
			btn2.onclick = fun1;
			function fun2(){
				console.log("funtion 2");
			}
			function fun3(){
				console.log("funtion 3");
			}
			btn3.addEventListener('click',fun1);
			btn3.addEventListener('click',fun2);
			btn3.addEventListener('click',fun3);
		</script>
	</body>
</html>